Auditory and vestibular disorders result from irreversible damage to sensory hair cells that mediate hearing and balance. The goal of this project is to identify sources of new sensory hair cells (stem cells) in the zebrafish inner ear. Sensory hair cell production continues throughout the life of fish and damaged hair cells are readily regenerated. We will test the hypothesis that embryonic sensory progenitor cells are maintained during postembryonic growth and provide a persistent source of hair cells in the adult zebrafish sensory epithelia. We will: A) Determine the embryonic source of sensory hair cells, B) Determine the postembryonic source of hair cells in a sensory epithelium, and C) Determine if embryonic sensory cell progenitors give rise to regeneration-competent cells in a mature sensory epithelium. The inner ears of zebrafish embryos and larvae are optically transparent and experimentally accessible. Experiments outlined in this proposal combine experimental embryology and state-of-the-art microscopy to identify hair cell progenitors in vivo. We will develop a high-resolution fate map of the zebrafish otic placode using two-photon dependant uncaging of caged fluorescein dextran to label single otic placode cells, trace their lineages during development and determine their ultimate fates. This fate map will identify the embryonic source of sensory hair cells. Using zebrafish larvae, we will determine the source of regenerated hair cells in the posterior crista during normal turnover and after laser ablation. We will investigate sensory cell population dynamics in vivo by two-photon imaging of cells in the posterior crista. Our efforts will culminate in a test of the hypothesis that sensory (stem) cell lineages are born in the otic placode, maintained throughout embryogenesis, and become progenitor cells in the larval ear. Identification and future characterization of sensory stem cells in the zebrafish will aid their identification in other vertebrates (most importantly mammals) with the ultimate goal of developing stem cell therapies to ameliorate the effects of deafness and balance disorders.